Proteins, which in addition to the desired protein, also have an undesirable constituent or "ballast" constituent in the end product are referred to as fusion proteins. When proteins are prepared by genetic engineering, the intermediate stage of a fusion protein is utilized particularly if, in direct expression, the desired protein is decomposed relatively rapidly by host-endogenous proteases, causing reduced or entirely inadequate yields of the desired protein.
The magnitude of the ballast constituent of the fusion protein is usually selected in such a manner that an insoluble fusion protein is obtained. This insolubility not only provides the desired protection against the host-endogenous proteases but also permits easy separation from the soluble cell components. It is usually accepted that the proportion of the desired protein in the fusion protein is relatively small, i.e. that the cell produces a relatively large quantity of "ballast".
The preparation of fusion proteins with a short ballast constituent has been attempted. For example, a gene fusion was prepared which codes for a fusion protein from the first ten amino acids of .beta.-galactosidase and somatostatin. However, it was observed that this short amino acid chain did not adequately protect the fusion protein against decomposition by the host-endogenous proteases (U.S. Pat. No. 4,366,246, Column 15, Paragraph 2).
From EP-A 0 290 005 and 0 292 763, we know of fusion proteins, the ballast constituent of which consists of a .beta.-galactosidase fragment with more than 250 amino acids. These fusion proteins are insoluble, but they can easily be rendered soluble with urea (EP-A 0 290 005).
Although fusion proteins have been described in the art, the generation of fusion proteins with desirable traits such as protease resistance is a laborious procedure and often results in fusion proteins that have a number of undesirable characteristics. Thus, a need exists for an efficient process for producing fusion proteins with a number of attractive traits including protease resistance, proper folding, and effective cleavage of the ballast from the desired protein.